The invention relates to a method and apparatus for producing a surface-structured, film-like semi-finished product made of thermoplastic which is applied onto a prepared surface with variable temperature that serves as a surface structure negative (matrix) for the desired product, in melted or film state, whereby the surface has low adhesion to the plastic material and is provided with numerous fine cavities. The surface is maintained at the melting temperature at least in the areas in contact with the plastic. The shaped, thermoplastic plastic material is solidified by cooling while still in contact with the surface whereby it assumes the corresponding surface structure. The hardened plastic material is removed from the surface as a structured film, whereby the thermoplastic material which is inserted into the cavities forms a pile comprised of projections and naps. The invention is related further to products manufactured by this process, and to a device to perform the process.
From the German Patent No. DE 195 24 076, manufacture of surface-structured, film-like semi-finished products made of thermoplastic is known in that the thermoplastic material is extruded in melted form by a cylindrical, rotatable roller surface whose temperature may be adjusted, whereby the thermoplastic material is in firm contact with the surface structure of the roller surface. The melted thermoplastic material is solidified by cooling, and is removed from the roller surface. During this process, a napped or crepe surface of the semi-finished film is produced that corresponds to the fine cavities. Pressing or deep drawing of plastic films to shape them is also known.
It has proven very difficult to provide fine projections and nap (for example, on the order of 3,000 to 20,000 units per cm2) to the upper surface of plastic film using the above-named process. It is also very difficult to extract the film projections from the deep cavities of the tool. Experience has shown that a certain portion of the film projections remain trapped in the cavities, so that they fall out during the next formation cycle, rendering the resulting product unusable after a short time.
Accordingly, the object of the invention is to produce a semi-finished product made of thermoplastic using a casting or application process without tearing off the small projections or nap in the cavities, but rather for which a sealed surface may be produced uniformly in several working steps.
This object, as well as other objects which will become apparent from the discussion that follows, are is achieved, according to the invention, using a process that comprises the following steps:
A thermoplastic material is applied in melted form or in the form of a film to a surface with adjustable temperature which forms a surface structure negative (matrix) to the final structure desired and which has low adhesion to the plastic material and numerous fine cavities in the form of blind holes. This surface is maintained at the melting temperature of the plastic at least in the contact area.
By pressure on the plastic material, this material is forced into the cavities under compression of the volume remaining in the cavities, so that the matrix is filled, but the cavities are only partially filled by the thermoplastic material.
The formed thermoplastic material is solidified by cooling while still on the surface, whereby it assumes a surface pattern corresponding to that of the surface with which it was in contact.
The pressure on the plastic material is released, whereby the air compressed within the cavities at least partially forces the plastic material out.
The solidified plastic material is removed from the surface as a structured film, whereby the thermoplastic material forced into the cavities and extracted from them forms a crepe surface consisting of projections and naps.
The plastic material is forced into the cavities under compression of the volume remaining in the cavities particularly by pressure, whereby over-pressure of from 2 to 5 bar may be used, so that the matrix is filled but the cavities are only partially filled by the thermoplastic material. When the pressure on the plastic material is released, the air compressed within the cavities expands and partially forces the plastic material out of the cavities. Since the projections and naps are connected with the other plastic materials, it is now easy to extract the projections normally subject to tearing.
This presupposes that the matrix is of a nature that it possesses a low degree of adhesion to the plastic material. Such matrices may be formed as a single piece from PTFE plastic according to known procedures.
A basic version of such rollers is described in the German Patent No. DE 195 24 076. Inclusion of fine cavities may be performed by a laser drill, for example. Rollers with steel cores covered by a 0.5xcx9c10 mm thick coating of fluoroplastic are particularly suited to this. Such fluoroplastic might be polyfluoroethylene or a fluoro-rubber such as products marketed by Du Pont under the trademark VITON(trademark), for example. The plastic layer must be able to withstand long-term roller temperatures of 200xc2x0 C. to 250xc2x0 C. Other suitable plastic roller coatings may be selected from the polyamide or polysulfone groups. Rollers made completely of steel or of steel with a metal or alloy coating may also be used.
For plastic materials to be treated, known thermoplastics may be used which include polyolefins, polyisopropylenes, polyesters, polyvinylalcohols, polyurethanes, polyesterethers, polyamides, polyvinyl chlorides, polysulfones, polycarbonates, ABS, ASA, polyesteramides, or mixtures or co-polymers thereof.
Removal of the structured film occurs at a temperature of 40xc2x0 C. to 60xc2x0 C., so that the film possesses a somewhat soft and easily-treated consistency. The temperature itself that enables the casting or melting of the plastic may be determined based on information from manufacturers, and may vary from case to case.
The pressure on the plastic material is preferably applied in a roller gap, i.e., by a roller pressing on the plastic material.
The length of the projections and naps is preferably between 50 and 200 xcexcm. For this, the length of the projections and naps represents 20% to 60% of the depth of the cavities. The density of projections or hair fibers is preferably between 3,000 and 20,000 units per cm2.
The process may be considerably expanded by a step in which the projections forming the crepe surface are processed by combing, brushing, raking, and/or scissoring, thereby increasing the length of the crepe finish projections on an average to at least twice their normal length, producing a semi-finished product with a structured fiber finish on at least one side for which the projections are lengthened into hair fibers.
A device to perform the process according to the invention includes a pair of rollers from which the one roller (1) is adjustable in temperature and possesses a surface worked to produce a negative structure (matrix) of the desired structure, (2) possesses low adhesive qualities to the plastic material, and (3) has numerous fine cavities in the shape of blind holes. At least a portion of the roller surface may be heated to the melting temperature of the thermoplastic material to be used. The plastic material may be pressed into the structure of the heated roller by a second roller of the pair, preferably also heated. Further, a cooling device is provided to cool the roller surface and the plastic material applied to it after it has passed through the roller gap, as is a stripping device to remove the solidified structured film.
The process normally involves two rollers, therefore, of which the matrix roller is left at a temperature of 40xc2x0 C. and the second smooth steel roller, for example with polyethylene, is kept at a temperature of 140xc2x0 C. xc2x15xc2x0 C. The steel roller heats the plastic film and radiant heat also softens it and heats it to 145xc2x0 C. The plastic material is pressed into the matrix and the cavities in the roller gap, and then is cooled there. Further characteristics of the device used to perform the process are explained below using embodiment examples.
Products produced by this process may be manufactured as mono- or multi-layer products or as a semi-finished or finished product with lengthened fibers.
It is possible that the layer provided with a crepe surface may consist of a lighter freely-flowing (when heated) polymer produced from a highly-viscous polymer.
Also, the side opposite to the fiber crepe finish might be bonded to a carrier material such as fabric, woven, or non-woven material.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.